helo: new plugin

helo - validate a HELO message delivered from a connecting host.

Includes the following tests:

	is_in_badhelo
	invalid_localhost
	is_plain_ip
	is_address_literal [N.N.N.N]
	is_forged_literal
	is_not_fqdn
	no_forward_dns
	no_reverse_dns
	no_matching_dns
helo: loosen up matching DNS requirements

added X-HELO header to message
added timeout option
quieted down debug logging
helo: tweak POD language

also mention the connection notes in the POD
Merge pull request #29 from msimerson/helo

Helo